1. Field of the Invention
The present invention relates to a lubricating oil for compression-type refrigerators and a novel polyoxyalkylene glycol derivative. More particularly, it relates to a lubricating oil for compression-type refrigerators having high lubricating property as well as good miscibility with hydrofluorocarbons (HFC) including 1,1,1,2-tetrafluoroethane (hereinafter referred to as R-134a), which can substitute for chlorofluorocarbons (CFC) including dichlorodifluoromethane (hereinafter referred to as R-12) used as a refrigerant and concerned for enviromental pollution problems, and a novel polyoxyalkylene glycol derivative effective as said lubricating oil. In the present specification, the term "hydrofluorocarbons (HFC)" means hydrofluorocarbons (HFC) including R-134a and hydrochlorofluorocarbons (HCFC) including R-22.
2. Description of the Related Arts
Generally, a compression-type refrigerator is composed of a compressor, a condenser, expansion valve, and an evaporator, having a mechanism wherein the mixture of a refrigerant and a lubricating oil is circulating in the closed system. In said compression-type refrigerator, though it depends on the kind of apparatus, generally the temperature in the compressor rises to 50.degree. C. or higher, while in the cooler, the temperature comes to be -40.degree. C. or so. Accordingly, the refrigerant and the lubricating oil must circulate in this system without phase separation usually in the range of -40.degree. to +50.degree. C. If phase separation occurs while the refrigerator is running, it affects the life and efficiency of the apparatus seriously. For example, if phase separation of the refrigerant and the lubricating oil occurs in the compressor, the moving parts would be inadequately lubricated, resulting in seizure or other troubles and thereby the life of apparatus is shortened considerably. If phase separation occurs in the evaporator, a lubricating oil having high viscosity exists and thereby the efficiency of heat exchange is decreased.
Since a lubricating oil for refrigerators is used for the purpose of lubricating the moving parts of the refrigerator, its lubricating property is also important as a matter of course. Since the temperature becomes very high, particularly in the compressor, it is required to have a sufficient viscosity to retain the oil film necessary for lubricating. Necessary viscosity varies with the kind or running conditions of the compressor, but usually, the kinematic viscosity of the lubricating oil before mixing with a refrigerant is preferably 2 to 50 cSt at 100.degree. C. If the kinematic viscosity is lower than the above, the oil film becomes thinner and thereby seizure is liable to arise, while if it is higher, the efficiency of heat exchange is decreased.
Heretofore, R-12 has often been used as the refrigerant for compression-type refrigerators, and various mineral oils and synthetic oils have been used as the lubricating oil, satisfying the required properties described above. R-12, however, has recently been restricted more and more severely all over the world, for the concern of environmental pollution problems, that is depletion of the ozone layer. Therefore, hydrofluorocarbons including R-134a have come to be noticed as a new refrigerant. Said hydrofluorocarbons, particularly R-134a has little possibility of depleting the ozone layer and can substitute for R-12, with minimal changes in the structure of the conventional refrigerators. Accordingly, it is preferable as a refrigerant for compression-type refrigerators.
When hydrofluorocarbons including the above R-134a are employed as the refrigerant for compression-type refrigerators in place of R-12, the desirable lubricating oils come to be those having high miscibility with said hydrofluorocarbons including R-134a, and also having high lubricating property to satisfy the required properties described above. However, since the conventional lubricating oils which have been used with R-12 do not have good miscibility with hydroflurorocarbons including R-134a, a new lubricating oil suitable for said compounds is required. In this case, particularly in the air-conditioner for automobiles, it is required that the equipment be hardly changed on the substitution for R-12. Therefore it is not desirable to change widely the present equipment due to a lubricating oil. Accordingly, a lubricating oil having very favorable miscibility with hydrofluorocarbons including R-134a is required.
As lubricating oils having miscibility with R-134a, for example, Ulcon LB-165 and Ulcon LB-525 (trade name, both produced by Union Carbide Co., Ltd.) composed of polyalkyleneglycol have been known, and it was reported that these lubricating oils are miscible with R-134a in all proportions at low temperature of at least -50.degree. C. ("Research Disclosure", No. 17463 (October, 1978)). And also, oil compositions for refrigerators with a high viscosity employing polyoxypropyleneglycol monobutyl ether as a base oil have been known (Japanese Patent Publication No. 42119/1982).
These lubricating oils, however, are polyalkylenenglycol derivatives having polypropyleneglycol with hydroxyl group at one terminal and an n-butyl ether bond at the other terminal. They have comparatively good miscibility with R-134a at low temperatures, but they do not have sufficient miscibility with R-134a at high temperatures, and for example, Ulcon LB-525 described above is known to cause phase separation with R-134a at room temperature (Specification of U.S. Pat. No. 4,755,316).
On the other hand, polyoxyalkylene glycol having at least two hydroxyl groups in a molecule is proposed to be a favorable substance miscible with R-134a (Specification of U.S. Pat. No. 4,755,316). In said polyoxyalkylene glycol, however, its miscibility is not necessary sufficient.
Meanwhile, it is known that polyoxyalkylene glycol generally shows a temperature dependency that the mixture thereof with hydrofluorocarbons, which has been phase-separated, is one dissolved and phase-separated again when heated from low temperatures to high temperatures. It is also known that the miscibility is lowered as the molecular weight of polyoxyalkylene glycol increases.
On the other hand, R-134a and compounds which can dissolve it were proposed for use in absorption-type refrigerators (Japanese Patent Application Laid-Open No. 79175/1981). Said absorption-type refrigerators, however, are quite different in mechanism from the compression-type refrigerators described above, and tetraethylene glycol dimethyl ether described in the Examples of the above Application is not proper as a lubricating oil for compression-type refrigerators because of its particularly low viscosity.
As described above, lubricating oils for compression-type refrigerators having sufficiently good miscibility with R-134a and high lubricating property have not been found yet, and their development has been eagerly desired.